Typically, radar apparatuses, e.g., mounted on a ship, are configured so as to transmit a pulse-shaped transmission signal from a rotating antenna and receive an echo signal from a target object, such as another ship. With this configuration, a distance to the target object can be detected by measuring a time length after transmitting the transmission signal until receiving the echo signal. Since the antenna performs the transmission of the transmission signal and the reception of the echo signal while rotating, the radar apparatus can detect target objects in arbitrary directions.
Such a radar apparatus may receive transmission signals from other radar apparatuses (typically, mounted on other ships) and, thus, those transmission signals may interfere with echo signals, degrading performance of the radar apparatus. Therefore, for such a type of radar apparatus, various techniques to remove the transmission signals from other radar apparatuses (i.e., interference signals) have been proposed.
Example processing of the radar apparatus for removing the interference signals is described with reference to FIGS. 9A and 9B. FIGS. 9A and 9B are graphs showing conventional interference removing processes. In FIGS. 9A and 9B, a vertical axis shows a distance from the radar apparatus, and a horizontal axis shows an azimuth when transmitting a transmission signal. The azimuth is indicated using a horizontal angle with respect to a predetermined bearing. Dashed lines parallel to the vertical axis indicate reception data for one sweep, each corresponds to the azimuth (i.e., an angle θn), and a bar with a hatched area which overlaps with the corresponding dashed line indicates an interference signal, respectively.
The radar apparatus performs, as processing for removing the interference signals, processing for changing a time interval for transmitting the transmission signal (i.e., a transmission cycle) each time it transmits the transmission signal. Thus, the radar apparatus receives the echo signals at timings corresponding to the transmission time intervals which are individually set to the radar apparatus. Contrary, the radar apparatus receives the interference signals from other radar apparatuses at different timings which correspond to transmission time intervals individually set to the other radar apparatuses. For this reason, the receiving timings of the necessary echo signals and the unnecessary interference signals will never have a fixed relation therebetween, and, therefore, the interference signals which the radar apparatus receives appear at random locations in the distance direction, as shown in FIG. 9A.
Note that, since the echo signals are reflection waves, their signal intensities are comparatively low; however, since the interference signals are received directly from other radar apparatuses, their signal intensities are comparatively high. Using this tendency, the interference signals can be removed by comparing reception data of one sweep with reception data of a previous or later sweep at the same distance, and using reception data with a lower signal intensity as an output value of the sweep of interest. For example, in FIG. 9A, when a total of three sweeps at angles θ1, θ2 and θ3 are observed, signal intensities of the reception data of the sweeps at the angles θ1 and θ3 are low near a distance r1, as compared with the reception data at the angle θ2. Therefore, as for the range near the distance r1, the interference signals can be removed by using the reception data of the sweep at the angle θ1 or the angle θ3 as an output value of the sweep at the angle θ2.
In recent years, the pulse compression radar apparatuses which transmit a modulated pulse signal using a semiconductor amplifier alternatively or additionally to a magnetron has just begun to be put in practical use for ship radars (see JP2008-96337A). The pulse compression radar apparatus transmits a transmission signal having a considerably long time width (i.e., a long pulse width) compared to the radar apparatus just using the magnetron. Then, by filtering the echo signals in accordance with the transmission signals, the pulse widths of the echo signals can be compressed and a signal-to-noise power ratio (i.e., S/N ratio) can be improved.
However, when filtering interference signals from other pulse compression radar apparatuses (i.e., signals with long pulse widths), pulse widths of the interference signals will not be compressed if modulation modes of the interference signals of other radar apparatuses differ from a modulation mode of the transmission signals of the radar apparatus of interest. Moreover, when filtering interference signals from other radar apparatuses using the magnetron (i.e., signals with short pulse widths), pulse widths of the interference signals will be large. In other words, when the pulse compression radar apparatus receives the interference signals, the interference signals appear to be elongated in the distance direction, as shown in FIG. 9B.
Therefore, it may be said that, regardless of changing the transmission time interval of the transmission signal such that the interference signals differ from each other in the distance direction, when the reception data are compared at the same distance, as described above, the interference signals continue over many sweeps in the azimuth direction (e.g., five sweeps from the angle θ1 to the angle θ5 near at a distance r2, as shown in FIG. 9B). As for this case, the interference signals cannot be properly removed by using the method of extracting about three adjacent sweeps and simply measuring the intensities of the signals. Thus, in the conventional pulse compression radar apparatus, it is difficult to remove the interference signals by a simple comparison of the signal intensities between sweeps.